The forward converter is basically a buck-type voltage regulator with transformer isolation between the power switch and the output filter inductor. When the power switch is conducting, current flows through a power transformer, a rectifying diode and the output filter inductor to the load. During nonconduction of the power switch, the rectifying diode is back biased; however, a load current due to stored energy in the filter inductor continues to flow through a flyback diode. Due to the unidirectionality of current flow, the build up of magnetizing flux in the primary winding is in only one direction and hence provision must be made to allow the core to reset by allowing the magnetizing flux to return to the initial state after each cycle.
The conventional methods of demagnetizing the transformer core have provided a discharge path for demagnetizing current when the power switch is nonconducting. One such embodiment utilizes a demagnetizing winding tightly coupled to the primary winding to conduct the demagnetizing current back to the voltage source. Other related techniques use a dissipative clamp to dissipate the energy stored in the transformer's core each cycle and must handle a very high average power level. These conventional techniques, however, either require a precision transformer winding or else significantly reduce the overall efficiency of the converter. Further details concerning these prior methods may be found in "A Design For Optimizing The Power Device Utilization In Feed-Forward Converters" by S. Hayes in Proceedings of Power Con 8, 1981, pages F-3; 1-10.
Another recent technique of resetting the transformer of a forward converter is disclosed in U.S. Pat. No. 4,441,146, issued to P. Vinciarelli on Apr. 3, 1984 and entitled "Optimal Resetting Of The Transformer's Core In Single Ended Forward Converters". This reference discloses a reset circuit in which a capacitor and an auxiliary transistor switch are connected in series. This series connection is in turn connected in parallel with the power transformer's secondary winding. A control circuit operates the auxiliary switch in an inverse synchronous relation with the power transistor switch so that when one conducts the other is nonconducting. The capacitor is sized such that its voltage is essentially constant during the reset period. During reset of the core when the auxiliary transistor switch is conducting, the magnetizing currents are initially utilized to charge the capacitor; subsequently the capacitor is discharged into the transformer winding supplying energy to reset it for the next cycle.
All of the above arrangements share the same disadvantage of requiring multi-component added reset circuitry which detracts from the otherwise natural circuit simplicity of the forward converter.